It is common practice to utilize a number of control devices to selectively operate various portions of a pressure system. In a hydraulic power plant having a plurality of operating cylinders, it is conventional to employ magnet valves to control the supply of hydraulic fluid pressure to the individual operating cylinders. In the past, each of the individual magnet valves was hard-wire connected by a separate communication link to a source of d.c. supply voltage. It will be appreciated that such a multichannel system is relatively expensive to construct and maintain.